This invention relates to a hollow cathode array for use in the creation of a discharge plasma. The plasma generated can be used to modify surface properties of substrates, such as films, fibers, particles and other articles.
Treatment of various substrates, such as polymers, to impart new surface properties through physical or chemical modification is important for many industries, including the film industry. Wet methods have successfully been used for such treatment; however, such wet methods are associated with problems, such as waste disposal of solvents. Dry methods, such corona treatments, uv treatments, laser treatments, x-ray and gamma-ray treatments have also been utilized with some success. Corona treatment has been in industrial use for several decades but is generally restricted to simple surface geometries, such as web structures. In addition, in some materials the effects of corona treatments fade-away with time. Further, there is little control over what functional groups may end up on the surface of a treated substrate, and the close distance between electrode and substrate can lead to undesirable formation of pin-holes or burn spots. UV, x-ray, gamma-ray and laser treatments are point sources and cannot readily be used to treat large areas. In addition, these treatments are subject to intensity variations or shading effects which may result in some regions receiving less treatment or even being blocked by line-of-sight shadowing.
Treatment of rigid, shaped or molded polymeric containers to impart improved surface properties and gas barrier properties is important to the food and beverage industries. Various application methods have been proposed to coat such containers with a variety of compositions and thereby improve their gas barrier properties. However, there continues to be a need to further increase the gas barrier properties of such containers to make them capable of better retarding the transmission of gases, such as oxygen and carbon dioxide. There also continues to be a need for improved surface properties, especially related to printability, to improve the recycleability of such containers.
Plasma technology has been used in the laboratory for more than 50 years, but it has only recently been practiced on a commercial scale, mainly driven by the semiconductor industries. In the plasma treatment of polymers, energetic particles and photons generated in the plasma interact strongly with the polymer surface, usually via free radical chemistry.
A major advantage of plasma surface treatment as compared with other treatment processes is the lack of harmful byproducts. There are usually no toxic or hazardous liquids or gases that must be disposed of. Usually, the main process byproducts for plasma treatment are CO, CO2, and water vapor, none of which is present in toxic quantities. Theoretically, plasmas can be applied to objects of all possible geometries with varying success using conventional apparatus and processes. Such objects include webs, films, large solid objects with complex shapes, and small discrete parts in large quantities, such as powder.
A major impediment to utilizing plasma processes on an industrial scale is achieving economically acceptable rates of production. The usefulness of plasma processes is readily apparent but the throughput is so low that the processes are economically feasible only for products which acquire greatly enhanced value from the process. Limiting factors include low plasma density and lack of plasma confinement. A plasma modification or polymerization system with a capability for high production rates would lead to rapid growth in the utilization of plasma technology on an industrial scale.
Most devices used to generate plasmas for plasma polymerization and plasma modification are variations of two basic types of electrode configurations: internal parallel plate electrodes and external electrodes. The usefulness of these two processes is limited by the ease in which they can be scaled up to treat large areas while maintaining high plasma density such that minimum residence time can be achieved. The existing DC hollow cathode plasma reactors offer higher plasma density and a higher degree of plasma confinement than the internal parallel plate and external electrodes. However, they cannot be used to treat large areas because a large hollow cathode reactor is inherently difficult to scale up. In order to achieve a desirable plasma density with a large hollow cathode reactor necessary to accommodate large area treatments, such as 60xe2x80x3 (152.4 cm) width substrates, an extremely high voltage is required.
EP 634 778 describes an array of hollow cathodes which generates plasma for surface etching and removal of xe2x80x9crolling oilsxe2x80x9d on metal sheets. The hollow cathode array system comprises a housing having a plurality of uniformly spaced openings along one wall of the housing. The hollow cathode plasma is basically generated in the housing and transported through the openings. The array of such openings serve as a distributor, however plasma intensity and uniformity are not enhanced. Each opening is about {fraction (1/16)} inch (0. 15 cm) in diameter. The pressure used in the system of EP""778 is within the range of 0.1 to 5.0 torrs (13.33 to 666.61 Pascal), and the power input is in the range of 0.5 to 3.0 kW.
U.S. Pat. No. 5,686,789 describes a discharge device having a cathode with a micro hollow array for use in subminiature fluorescent lamps. This patent covers devices with dimensions on the scale of the mean free path of electrons which is not viable for large area treatments. The electrons undergo oscillatory motion within the micro hollows which produces a micro hollow discharge resulting in increased current capability. In use, the system of U.S. ""789 uses a pressure of 0.1 torr to 200 torrs (13.33 to 26,664.48 Pascal). The patent does not mention reactive plasma, plasma polymerization, or surface modification of materials.
H. Koch et al. in an article entitled xe2x80x9cHollow cathode discharge sputtering device for uniform large area thin film deposition,xe2x80x9d (J.Vac. Sol. Technol. A9 (4), July/August 1991, p. 2374) describe hollow cathode discharge devices which produce higher densities of sputtered particles than conventional discharges. In the hollow cathode plasma sputtering process described therein, a sputtered target, e.g. Cu, is used as the cathode. The process is primarily a physical process in which momentum transfer takes place. The article does not mention using hollow cathode discharge devices in creating reactive plasma for surface; modification or conducting plasma polymerization for depositing a thin layer on substrate surfaces.
There continues to be a need for a process for surface treatment that among others (1) is solventless and/or free of harmful by-products, (2) is versatile in tailoring the treatment to any surface structure, any size, and/or chemistry of a given article, (3) provides a uniform treatment, (4) is capable of a high rate and a high throughput, (5) can be used in a batch or continuous process, and (6) can be operated at low pressure or under other desirable conditions.
The present invention concerns a cathode assembly for generating a plasma comprising a plurality of electrically conductive hollow plasma generating cells in an array, the cells being electrically connected to each other.
The present invention also concerns a plasma generating apparatus, comprising at least one cathode assembly described above; means for supplying a plasma precursor gas to the cathode assembly; and means for supplying power to the cathode assembly.
The present invention further concerns a method of treating a surface of a substrate comprising positioning the substrate in close proximity to at least one cathode assembly described above; supplying at least one plasma precursor gas to the vicinity of the cathode assembly and the substrate; generating a plasma by supplying power to the cathode assembly; and exposing the surface of the substrate to the plasma for a time sufficient to form a treated surface.
The present invention also concerns a method for packaging a liquid in a molded biaxially oriented polymeric container comprsing (a) forming a molded biaxially oriented polymeric container; (b) exposing at least one surface of the container to a plasma generated from a plasma precursor gas comprising a hydrocarbon using the cathode assembly of the present invention described herein, wherein the array is shaped; (c) introducing a liquid into the container; and (d) sealing the container.
The present invention further concerns a method for reducing the gas permeability of a polyester substrate comprising exposing a polyester substrate to a plasma generated from a plasma precursor gas comprising a hydrocarbon using the cathode assembly of the present invention described herein.